lips are sealed表示他的口会像信封或者瓶子一样，被封住，引申为守口如瓶；把嘴封住；绝口不提。

【例句】
I told him not to say anything, and he said his lips are sealed.
我告诉他不要把任何事情说出去，而他也表示他会守口如瓶。

My lips are sealed, I won't tell you.
我现在守口如瓶，不会告诉你的。